The present invention relates to the structure of a pressure detection gage for a semiconductor pressure sensor for detecting a pressure difference or a pressure.
As a conventional semiconductor pressure sensor of this type, a sensor using an Si (silicon) semiconductor diaphragm is known. This Si diaphragm type semiconductor pressure sensor is formed as follows. A gage serving as a piezoelectric region is formed on the upper surface of a semiconductor substrate by diffusion of an impurity or ion implantation. In addition, leads are formed by vapor deposition of Al or the like. Part of the lower surface of the substrate is then etched to form a distortion portion having a thickness of about 20 .mu.m to 50 .mu.m, i.e., a diaphragm. In the pressure sensor formed in this manner, when measurement pressures are respectively applied to the upper and lower surfaces of the diaphragm, the resistivity of the gage changes upon deformation of the diaphragm. By detecting an output voltage accompanying this change in resistivity, a pressure difference or a pressure is measured. The piezoelectric resistance coefficients decrease with an increase in amount of an impurity doped into the semiconductor substrate regardless of whether the impurity is of p type or n type. For this reason, in order to increase the rate of change in resistivity of the gage to improve sensitivity with respect to pressure, the concentration of an impurity is set to be low. In addition, the piezoelectric resistance coefficients change depending on whether a p-type or n-type impurity is used. The piezoelectric resistance coefficients are larger when a p-type impurity is used than when an n-type impurity is used. For this reason, a p-type resistive layer is generally formed on an n-type semiconductor.
FIG. 6 shows a gage in a conventional semiconductor pressure sensor. FIG. 7 shows a cross-section of the gage along a line VII--VII in FIG. 6. Referring to FIGS. 6 and 7, reference numeral 1 denotes an n-type semiconductor substrate consisting of an Si single crystal; and 2, a p-type folded gage formed on the upper surface of the semiconductor substrate 1 and serving as a piezoelectric resistive region. This folded gage 2 is constituted by a pair of gage portions 2a and 2b formed parallel to each other and having a predetermined sheet resistance and a low impurity concentration (10.sup.19 /cm.sup.3), a coupling portion 3 coupling one end of the gate portion 2a to that of the gate portion 2b, and a pair of lead out portions 4a and 4b to which the other end of the gage portion 2a and that of the gage portion 2b are respectively connected. In general, the coupling portion 3 and the lead out portions 4a and 4b are formed from a p.sup.+ -type semiconductor region having a high impurity concentration (10.sup.21 /cm.sup.3) in order to eliminate the influences of these components on the gage portions 2a and 2b. Reference numerals 5a and 5b denote aluminum leads formed by vapor deposition. End portions of these leads 5a and 5b are respectively connected to the lead out portions 4a and 4b.
When a reverse voltage is applied to a p-n junction, a saturation current Is flows only slightly. When, however, the reverse voltage exceeds a predetermined voltage, a large current flows abruptly. This phenomenon is called breakdown, and this predetermined voltage is called a breakdown voltage VB. If a voltage exceeding the breakdown voltage VB is applied to a p-n junction, the p-n junction mechanically breaks down for the following reason. As the reverse voltage applied to the p-n junction becomes higher, the electric field becomes stronger to increase the energies of electrons and holes. When such electrons collide with other electrons, the valence electron bonds are destroyed to abruptly cause electron avalanche. Therefore, the breakdown voltage VB is preferably set to be high. However, since the coupling portion 3 and the lead out portions 4a and 4b are formed from a heavily doped p.sup.+ -type semiconductor region having a low resistance, the depletion layer width is small, resulting in a strong electric field. For this reason, in the coupling portion 3 and the lead out portions 4a and 4b, the reverse withstand voltage of the p-n junction is low, and the breakdown voltage VB is low.